The present invention relates generally to heat exchanging devices. More particularly, the invention relates to techniques for monitoring thermal performance of heat exchangers, analyzing reasons for changes in performance over time, and ameliorating performance.
Heat exchanging systems are employed across a wide range of applications and industries. In general, such systems serve to transfer thermal energy between two process fluids. The fluids may be of many different types, and many systems employ water or steam for at least one of the fluids. The direction of thermal transfer is typically determined based upon which fluid is to be heated or cooled in the particular application. In practice, the fluids may undergo sensible heat changes (i.e. exhibiting changes in temperature), latent heat changes (i.e. causing changes in phase), or both.
Many different types of heat exchangers are known and in use. For example, in one common design tubes extend from one end of a shell to another to establish one or more passes of one fluid through the other. One of the fluids is then routed though the tubes, while the second is circulated through the shell. The tubes serve to isolate the fluids from one another and to transfer thermal energy between the fluids. The rate of heat transfer depends on factors such as the flow rate of the fluids, their inlet and outlet temperatures, individual heat transfer coefficients, over all heat transfer coefficients etc. Other types of heat exchangers operate on different principles, such as evaporation or condensation (i.e. phase change) of one or both fluids.
Design parameters for heat exchangers are typically determined on an application-specific basis. That is, based upon the needs for thermal transfer, the fluids to be heated and cooled, environment within which the systems will operate, and the desired life of the equipment, desired material, styles and operating specifications are determined. Moreover, design parameters generally assume ranges of tolerance in operating conditions and performance, including the efficiency and rates of heat transfer between the circulating fluids.
One difficulty that arises in heat exchanger systems is the loss of the heat transfer capabilities over time. Reduction in the rate of heat transfer may result from a number of root causes, and is often related to fouling of the exchanger paths and heat transfer surfaces. Underlying causes of fouling may include such factors as deposition of materials within the flow paths or on the heat transfer surfaces, chemical reactions within the exchanger, precipitation of materials, particulate matter within the exchanger, corrosion of the exchanger materials, biological growth or deposition, and so forth.
Certain approaches have been developed to characterize such fouling and to avoid it. For example, certain factors have been tracked as indicators of fouling so as to permit servicing when performance falls below desired levels. In systems in which water constitutes one of the process streams, the water is typically treated with chemicals to prevent or to reduce the occurrence of chemical deposition, chemical reactions, and so forth. However, such approaches have been somewhat limited in their ability accurately to characterize the causes of fouling, and they do not provide adequate tools for evaluating trends, broadly diagnosing system factors leading to fouling, or prognosticating changes that could improve efficiency, reduce downtime for servicing, and avoid or reduce related costs. Many current systems are simply inadequate due to insufficient monitoring of process parameters needed to generate early warnings of impending problems, the inability to diagnose causes of degradation or failures, and the lack of diagnostic and predictive know-how to tie the correct diagnosis to effective corrective actions.
There is a need, therefore, for improved techniques for monitoring and characterizing heat exchanger performance. The need is particularly prevalent, in that heat exchangers are found in such a wide variety of industries, including chemical plants, polymer processes, air separation plants, refineries, hotel chains and building management concerns, to name but a few. Consequences of failing to accurately control heat exchanger performance include high energy consumption, loss of production capacities, increased occurrences of shut-downs, and cleaning costs. Moreover, in extreme cases, failure of the heat exchanger may result, causing rupture and leaks, resulting in environmental concerns and equipment maintenance or replacement costs.